Photosensitized oxidation, by single-electron transfer, of catharanthine and vindoline: a highly regio- and diastereoselective photocyanation reaction

The irradiation, by visible light, of (+)-catharanthine 1 and of (-)-16-O-acetylvindoline 5 in the presence of cyanide ion and a catalytic amount of a photosensitizer known to produce singlet oxygen leads to (+)-3 beta-cyanocatharanthine 3 and the new (-)-16-O-acetyl-3 alpha-cyanovindoline 6. The complete stereostructural identification of these compounds has been ascertained by an accurate spectroscopic survey. High regio- and diastereoselectivities (de > 99%) are observed. Some experiments have been performed which confirm unambiguously that singlet oxygen is the oxidizing species in such photochemical conditions.